Beyond the orbit of Neptune, the farthest recognized-planet from our Sun, lies the mysteries population known as the Trans-Neptunian Object (TNOs). For years, astronomers have been discovering bodies and minor planets in this region which are influenced by Neptune’s gravity, and orbit our Sun at an average distance of 30 Astronomical Units.
In recent years, several new TNOs have been discovered that have caused us to rethink what constitutes a planet, not to mention the history of the Solar System. The most recent of these mystery objects is called “Niku”, a small chunk of ice that takes its name for the Chinese word for “rebellious”. And while many such objects exist beyond the orbit of Neptune, it is this body’s orbital properties that really make it live up to the name!
In a paper recently submitted to arXiv, the international team of astronomers that made the discovery explain how they found the TNO using the Panoramic Survey Telescope and Rapid Response System 1 Survey (Pan-STARRS 1). Measuring just 200 km (124 miles) in diameter, this object’s orbit is tilted 110° to the plane of the Solar system and orbits the Sun backwards.
Ordinarily, when planetary systems form, angular momentum forces everything to spin in the same direction. Hence why, when viewed from the celestial north pole, all the objects in our Solar System appear to be orbiting the Sun in a counter-clockwise direction. So when objects orbit the Sun in the opposite direction, an outside factor must be at play.
What’s more, the team compared the orbit of Niku with other high-inclination TNOs and Centaurs, and noticed that they occupy a common orbital plane and experience a clustering effect. As Dr. Matthew J. Holman – a professor at the Harvard-Smithsonian Center for Astrophysics and one of the researchers on the team – told Universe Today via email:
“The orbit of Niku is unusual in that it is nearly perpendicular to the plane of the Solar System. More than that, it is orbiting in the opposite direction of most Solar System bodies. Furthermore, there are a few bodies that share the same or orbital plane, with some orbiting prograde and some orbiting retrograde. That was unexpected.”
One possibility, which the team has already considered, was that this mysterious orbital pattern might be evidence of the much sought-after Planet Nine. This hypothetical planet, which is believed to exist at the outer edge of our Solar System (20 times further from our Sun than Neptune), if it exists, is also thought to be 10 times the size of the Earth.
“Planet Nine seems to be gravitationally influencing another nearby population of bodies that are also orbiting nearly perpendicular to the plane of the solar system,” said Holman, “but those objects have much larger orbits that also come closer to sun at their closest approach. The similarity (perpendicular) nature of Niku’s orbit to that of the more distant population hints at a connection.”
Establishing such a connection based on the orbits of distant objects is certainly tempting, especially since no direct evidence of Planet Nine has been obtained yet. However, upon further analysis, the team concluded that Niku is too close to the rest of the Solar System for its orbit to be effected by Planet Nine.
In addition, the orbits of the clustered objects that circle the sun backwards along the same 110-degree plane path was seen as a further indication that something else is probably at work. Then again, it may very well be that there is a giant planet out there, and that it’s influence is mitigated by other factors we are not yet aware of.
“The population of objects in Niku-like orbits is not long-term stable,” said Holman. “We hoped that adding the gravitational influence of an object like Planet Nine might stabilize their orbits, but that turned out not to be the case. We are NOT ruling out Planet Nine, but we are not finding any direct evidence for it, at least with this investigation.”
So for the time being, it looks like Planet Nine enthusiasts are going to have to wait for some other form of confirmation. But as Konstantin Batyagin – the Caltech astronomer who announced findings that hinted at Planet Nine earlier this year – was quoted as saying, this discovery is yet another step in the direction of a more complete understanding of the outer Solar System:
“Whenever you have some feature that you can’t explain in the outer solar system, it’s immensely exciting because it’s in some sense foreshadowing a new development. As they say in the paper, what they have right now is a hint. If this hint develops into a complete story that would be fantastic.”
Whatever the cause of Niku’s strange orbit (or those TNOs that share its orbital pattern) may be, it is clear that there is more going on in the outer Solar System than we thought. And with every new discovery, and every new object catalogued by astronomers, we are bettering our understanding of the dynamics that are at work out there.
In the meantime, perhaps we’ll just need to send some additional missions out that way. We have nothing to lose but our preconceived notions! And be sure to enjoy this video about this latest find, courtesy of New Scientist:
After being officially discovered by Clyde Tombaugh in 1930, Pluto spent close to a century being thought of as the ninth planet of our Solar System. In 2006, it was reclassified as a “dwarf planet” due to the discovery of other Trans-Neptunian Objects (TNOs) of comparable size. However, that does not change its significance in our galaxy. In addition to being the largest TNO, it is the largest and second-most massive dwarf planet of our Solar System.
As a result, a great deal of time and study has been devoted to this former planet. And with the successful flyby of the New Horizons mission this month, we finally have a clear picture of what it looks like. As scientists pour over the voluminous amounts of data being sent back, our understanding of this world at the edge of our Solar System has grown by leaps and bounds.
The existence of Pluto was predicted before it was observed. In the 1840s, French mathematician Ubrain Le Verrier used Newtonian mechanics to predict the position of Neptune (which had not yet been discovered) based on the perturbation of Uranus. By the late 19th century, subsequent observations of Neptune led astronomers to believe that a planet was perturbing its orbit as well.
In 1906, Percival Lowell – an American mathematician and astronomer who founded the Lowell Observatory in Flagstaff, Arizona, in 1894 – initiated a project to locate “Planet X”, the possible ninth planet of the Solar System. Unfortunately, Lowell died in 1916 before a confirmed discovery was made. But unbeknownst to him, his surveys had captured two faint images of Pluto (March 19th and April 7th, 1915), which were not recognized for what they were.
After Lowell’s death, the search did not resume until 1929, at which point the director of the Lowell Observatory (Vesto Melvin Slipher) entrusted the job of locating Planet X to Clyde Tombaugh. A 23 year-old astronomer from Kansas, Tombaugh spent the next year photographing sections of the night sky and then analyzing the photographs to determine if any objects had shifted position.
On February 18th, 1930, Tombaugh discovered a possible moving object on photographic plates taken in January of that year. After the observatory obtained further photographs to confirm the existence of the object, news of the discovery was telegraphed to the Harvard College Observatory on March 13th, 1930. The mysterious Planet X had finally been discovered.
After the discovery was announced, the Lowell Observatory was flooded with suggestions for names. The name Pluto, based on the Roman god of the underworld, was proposed by Venetia Burney (1918–2009), a then eleven-year-old schoolgirl in Oxford, England. She suggested it in a conversation with her grandfather who passed the name on to astronomy professor Herbert Hall Turner, who cabled it to colleagues in the United States.
The object was officially named on March 24th, 1930, and it came down to a vote between three possibilities – Minerva, Cronus, and Pluto. Every member of the Lowell Observatory voted for Pluto, and the name was announced on May 1st, 1930. The choice was based on part on the fact that the first two letters of Pluto – P and L – corresponded to the initials of Percival Lowell.
The name quickly caught on with the general public. In 1930, Walt Disney was apparently inspired by it when he introduced a canine companion for Mickey Mouse named Pluto. In 1941, Glenn T. Seaborg named the newly created element plutonium after Pluto. This was in keeping with the tradition of naming elements after newly discovered planets – such as uranium, which was named after Uranus; and neptunium, which was named after Neptune.
Size, Mass and Orbit:
With a mass of 1.305±0.007 x 1o²² kg – which is the equivalent of 0.00218 Earths and 0.178 Moons – Pluto is the second most-massive dwarf planet and the tenth-most-massive known object directly orbiting the Sun. It has a surface area of 1.765×107 km, and a volume of 6.97×109 km3.
Pluto has a moderately eccentric and inclined orbit, which ranges from 29.657 AU (4.4 billion km) at perihelion to 48.871 AU (7.3 billion km) at aphelion. This means that Pluto periodically comes closer to the Sun than Neptune, but a stable orbital resonance with Neptune prevents them from colliding.
Pluto has an orbital period of 247.68 Earth years, meaning it takes almost 250 years to complete a single orbit of the Sun. Meanwhile, its rotation period (a single day) is equal to 6.39 Earth days. Like Uranus, Pluto rotates on its side, with an axial tilt of 120° relative to its orbital plane, which results in extreme seasonal variations. At its solstices, one-fourth of its surface is in continuous daylight, whereas another fourth is in continuous darkness.
Composition and Atmosphere:
With a mean density of 1.87 g/cm3, Pluto’s composition is differentiated between an icy mantle and a rocky core. The surface is composed of more than 98% nitrogen ice, with traces of methane and carbon monoxide. The surface is very varied, with large differences in both brightness and color. A notable feature is a large, pale area nicknamed the “Heart”.
Scientists also suspect that Pluto’s internal structure is differentiated, with the rocky material having settled into a dense core surrounded by a mantle of water ice. The diameter of the core is believed to be approximately 1700 km, 70% of Pluto’s diameter. Thanks to the decay of radioactive elements, it is possible that Pluto contains a subsurface ocean layer that is 100 to 180 km thick at the core–mantle boundary.
Pluto has a thin atmosphere consisting of nitrogen (N2), methane (CH4), and carbon monoxide (CO), which are in equilibrium with their ices on Pluto’s surface. However, the planet is so cold that during part of its orbit, the atmosphere congeals and falls to the surface. The average surface temperature is 44 K (-229 °C), ranging from 33 K (-240 °C) at aphelion to 55 K (-218 °C) at perihelion.
Pluto has five known satellites. The largest, and closest in orbit to Pluto, is Charon. This moon was first identified in 1978 by astronomer James Christy using photographic plates from the United States Naval Observatory (USNO) in Washington, D.C. Beyond Charon lies the four other circumbinary moons – Styx, Nix, Kerberos, and Hydra, respectively.
Nix and Hydra were discovered simultaneously in 2005 by the Pluto Companion Search Team using the Hubble Space Telescope. The same team discovered Kerberos in 2011. The fifth and final satellite, Styx, was discovered by the New Horizons spacecraft in 2012 while capturing images of Pluto and Charon.
Charon, Styx and Kerberos are all massive enough to have collapsed into a spheroid shape under their own gravity. Nix and Hydra, meanwhile, are oblong in shape. The Pluto-Charon system is unusual, since it is one of the few systems in the Solar System whose barycenter lies above the primary’s surface. In short, Pluto and Charon orbit each other, causing some scientists to claim that it is a “double-dwarf system” instead of a dwarf planet and an orbiting moon.
In addition, it is unusual in that each body is tidally locked to the other. Charon and Pluto always present the same face to each other; and from any position on either body, the other is always at the same position in the sky, or always obscured. This also means that the rotation period of each is equal to the time it takes the entire system to rotate around its common center of gravity.
In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers. This would seem indicate that Pluto does have a subsurface ocean that is warm in temperature, and that the core is geologically active. Pluto’s moons are believed to have been formed by a collision between Pluto and a similar-sized body early in the history of the Solar System. The collision released material that consolidated into the moons around Pluto.
From 1992 onward, many bodies were discovered orbiting in the same area as Pluto, showing that Pluto is part of a population of objects called the Kuiper Belt. This placed its official status as a planet in question, with many asking whether Pluto should be considered separately or as part of its surrounding population – much as Ceres, Pallas, Juno and Vesta, which lost their planet status after the discovery of the Asteroid Belt.
On July 29h, 2005, the discovery of a new Trans-Neptunian Object (TNO), Eris, was announced, which was thought to be substantially larger than Pluto. Initially referred to the as the Solar System’s “tenth planet”, there was no consensus on whether or not Eris constituted the planet. What’s more, others in the astronomic community considered its discovery the strongest argument for reclassifying Pluto as a minor planet.
The debate came to a head on August 24th, 2006 with an IAU resolution that created an official definition for the term “planet”. According to the XXVI General Assembly of the International Astronomical Union, a planet must meet three criteria: it needs to be in orbit around the Sun, it needs to have enough gravity to pull itself into a spherical shape, and it needs to have cleared its orbit of other objects.
Pluto fails to meet the third condition, because its mass is only 0.07 times that of the mass of the other objects in its orbit. The IAU further decided that bodies that do not meet criterion 3 would be called dwarf planets. On September 13th, 2006, the IAU included Pluto, and Eris and its moon Dysnomia, in their Minor Planet Catalog.
The IAUs decision was met with mixed reactions, especially from within the scientific community. For instance, Alan Stern, the principal investigator with NASA’s New Horizons mission to Pluto, and Marc W. Buie – an astronomer with the Lowell Observatory – have both openly voiced dissatisfaction with the reclassification. Others, such as Mike Brown – the astronomer who discovered Eris – have voiced their support.
On August 14th – 16th, 2008, in what came to be known as “The Great Planet Debate“, researchers on both sides of the issue gathered at Johns Hopkins University Applied Physics Laboratory. Unfortunately, no scientific consensus was reached; but on June 11th 2008, the IAU announced in a press release that the term “plutoid” would henceforth be used to refer to Pluto and other similar objects.
Pluto presents significant challenges for spacecraft because of its small mass and great distance from Earth. In 1980, NASA began to contemplate sending the Voyager 1 spacecraft on a flyby of Pluto. However, the controllers opted instead for a close flyby of Saturn’s moon Titan, resulting in a trajectory incompatible with a Pluto flyby.
Voyager 2 never had a plausible trajectory for reaching Pluto, but it’s flyby Neptune and Triton in 1989 led scientists to once again begin contemplating a mission that would take a spacecraft to Pluto for the sake of studying the Kuiper Belt and Kuiper Belt Objects (KBOs). This led to the formation of the Pluto Kuiper Express mission proposal, and NASA instructing the JPL to being planning for a Pluto, Kuiper Belt flyby.
By 2000, the program had been scrapped due to apparent budget concerns. After much pressure had been brought to bear by the scientific community, a revised mission to Pluto, dubbed New Horizons, was finally granted funding from the US government in 2003. New Horizons was launched successfully on January 19th, 2006.
From September 21st-24th, 2006, New Horizons managed to capture its first images of Pluto while testing the LORRI instruments. These images, which were taken from a distance of approximately 4,200,000,000 km (2.6×109 mi) or 28.07 AU and released on November 28th, confirmed the spacecraft’s ability to track distant targets.
Distant-encounter operations at Pluto began on January 4th, 2015. Between January 25th to 31st, the approaching probe took several images of Pluto, which were released by NASA on February 12th. These photos, which were taken at a distance of more than 203,000,000 km (126,000,000 mi) showed Pluto and its largest moon, Charon.
The New Horizons spacecraft made its closest approach to Pluto at 07:49:57 EDT (11:49:57 UTC) on July 14th, 2015, and then Charon at 08:03:50 EDT (12:03:50 UTC). Telemetries confirming a successful flyby and the health of the spacecraft reached Earth on 20:52:37 EDT (00:52:37 UTC).
During the flyby, the probe captured the clearest pictures of Pluto to date, and full analyses of the data obtained is expected to take years to process. The spacecraft is currently traveling at a speed of 14.52 km/s (9.02 mi/s) relative to the Sun and at 13.77 km/s (8.56 mi/s) relative to Pluto.
Though the New Horizons mission has shown us much about Pluto – and will continue to do so as scientists pour over all the data collected by the probe’s instruments – we still have much to learn about this distant and mysterious world. In time, and with more missions to the outer Solar System, we may eventually be able to unlock some of its deeper mysteries.
Until then, we offer all information that is currently available on Pluto. We hope that you find what you are looking for in the links below and, as always, enjoy your research!
Could there be another Pluto-like object out in the far reaches of the Solar System? How about two or more?
Earlier this week, we discussed a recent paper from planet-hunter Mike Brown, who said that while there aren’t likely to be any bright, easy-to-find objects, there could be dark ones “lurking far away.” Now, a group of astronomers from the UK and Spain maintain at least two planets must exist beyond Neptune and Pluto in order to explain the orbital behavior of objects that are even farther out, called extreme trans-Neptunian objects (ETNO).
We do know that Pluto shares its region Solar System with more than 1500 other tiny, icy worlds along with likely countless smaller and darker ones that have not yet been detected.
In two new paper published this week, scientists at the Complutense University of Madrid and the University of Cambridge noted that the most accepted theory of trans-Neptunian objects is that they should orbit at a distance of about 150 AU, be in an orbital plane – or inclination – similar to the planets in our Solar System, and they should be randomly distributed.
But that differs from what is actually observed. What astronomers see are groupings of objects with widely disperse distances (between 150 AU and 525 AU) and orbital inclinations that vary between 0 to 20 degrees.
“This excess of objects with unexpected orbital parameters makes us believe that some invisible forces are altering the distribution of the orbital elements of the ETNO,” said Carlos de la Fuente Marcos, scientist at UCM and co-author of the study, “ and we consider that the most probable explanation is that other unknown planets exist beyond Neptune and Pluto.”
He added that the exact number is uncertain, but given the limited data that is available, their calculations suggest “there are at least two planets, and probably more, within the confines of our solar system.”
In their studies, the team analyzed the effects of what is called the ‘Kozai mechanism,’ which is related to the gravitational perturbation that a large body exerts on the orbit of another much smaller and further away object. They looked at how the highly eccentric comet 96P/Machholz1 is influenced by Jupiter (it will come near the orbit of Mercury in 2017, but it travels as much as 6 AU at aphelion) and it may “provide the key to explain the puzzling clustering of orbits around argument of perihelion close to 0° recently found for the population of ETNOs,” the team wrote in one of their papers.
They also looked at the dwarf planet discovered last year called 2012 VP113 in the Oort cloud (its closest approach to the Sun is about 80 astronomical units) and how some researchers say it appears its orbit might be influenced by the possible presence of a dark and icy super-Earth, up to ten times larger than our planet.
“This Sedna-like object has the most distant perihelion of any known minor planet and the value of its argument of perihelion is close to 0°,” the team writes in their second paper. “This property appears to be shared by almost all known asteroids with semimajor axis greater than 150 au and perihelion greater than 30 au (the extreme trans-Neptunian objects or ETNOs), and this fact has been interpreted as evidence for the existence of a super-Earth at 250 au. In this scenario, a population of stable asteroids may be shepherded by a distant, undiscovered planet larger than the Earth that keeps the value of their argument of perihelion librating around 0° as a result of the Kozai mechanism.”
Of course, the theory put forth in two papers published by the team goes against the predictions of current models on the formation of the Solar System, which state that there are no other planets moving in circular orbits beyond Neptune.
But the team pointed to the recent discovery of a planet-forming disk around the star HL Tauri that lies more than 100 astronomical units from the star. HL Tauri is more massive and younger than our Sun and the discovery suggests that planets can form several hundred astronomical units away from the center of the system.
The team based their analysis by studying 13 different objects, so what is needed is more observations of the outer regions of our Solar System to determine what might be hiding out there.
Sometimes when you stare at something long enough, you begin to see things. This is not the case with optical sensors and telescopes. Sure, there is noise from electronics, but it’s random and traceable. Stargazing with a telescope and camera is ideal for staring at the same patches of real estate for very long and repeated periods. This is the method used by the Dark Energy Survey (DES), and with less than one percent of the target area surveyed, astronomers are already discovering previously unknown objects in the outer Solar System.
The Dark Energy Survey is a five year collaborative effort that is observing Supernovae to better understand the structures and expansion of the universe. But in the meantime, transient objects much nearer to home are passing through the fields of view. Trans-Neptunian Objects (TNOs), small icy worlds beyond the planet Neptune, are being discovered. A new scientific paper, released as part of this year’s American Astronomical Society gathering in Seattle, Washington, discusses these newly discovered TNOs. The lead authors are two undergraduate students from Carleton College of Northfield, Minnesota, participating in a University of Michigan program.
The Palomar Sky Survey (POSS-1, POSS-2), the Sloan Digital Sky Survey, and every other sky survey have mapped not just the static, nearly unchanging night sky, but also transient events such as passing asteroids, comets, or novae events. The Dark Energy Survey is looking at the night sky for structures and expansion of the Universe. As part of the five year survey, DES is observing ten select 3 square degree fields for Type 1a supernovae on a weekly basis. As the survey proceeds, they are getting more than anticipated. The survey is revealing more trans-Neptunian objects. Once again, deep sky surveys are revealing more about our local environment – objects in the farther reaches of our Solar System.
DES is an optical imaging survey in search of Supernovae that can be used as weather vanes to measure the expansion of the universe. This expansion is dependent on the interaction of matter and the more elusive exotic materials of our Universe – Dark Energy and Dark Matter. The five year survey is necessary to achieve a level of temporal detail and a sufficient number of supernovae events from which to draw conclusions.
In the mean time, the young researchers of Carleton College – Ross Jennings and Zhilu Zhang – are discovering the transients inside our Solar System. Led by Professor David Gerdes of the University of Michigan, the researchers started with a list of nearly 100,000 observations of individual transients. Differencing software and trajectory analysis helped identify those objects that were trans-Neptunian rather than asteroids of the inner Solar System.
While asteroids residing in the inner solar system will pass quickly through such small fields, trans-Neptunian objects (TNOs) orbit the Sun much more slowly. For example, Pluto, at an approximate distance of 40 A.U. from the Sun, along with the object Eris, presently the largest of the TNOs, has an apparent motion of about 27 arc seconds per day – although for a half year, the Earth’s orbital motion slows and retrogrades Pluto’s apparent motion. The 27 arc seconds is approximately 1/60th the width of a full Moon. So, from one night to the next, TNOs can travel as much as 100 pixels across the field of view of the DES survey detectors since each pixel has a width of 0.27 arc seconds.
The scientific sensor array, DECam, is located at Cerro Tololo Inter-American Observatory (CTIO) in Chile utilizing the 4-meter (13 feet) diameter Victor M. Blanco Telescope. It is an array of 62 2048×4096 pixel back-illuminated CCDs totaling 520 megapixels, and altogether the camera weighs 20 tons.
With a little over 2 years of observations, the young astronomers stated, “Our analysis revealed sixteen previously unknown outer solar system objects, including one Neptune Trojan, several objects in mean motion resonances with Neptune, and a distant scattered disk object whose 1200-year orbital period is among the 50 longest known.”
“So far we’ve examined less than one percent of the area that DES will eventually cover,” says Dr. Gerdes. “No other survey has searched for TNOs with this combination of area and depth. We could discover something really unusual.”
What does it all mean? It is further confirmation that the outer Solar System is chock-full of rocky-icy small bodies. There are other examples of recent discoveries, such as the search for a TNO for the New Horizons mission. As New Horizons has been approaching Pluto, the team turned to the Hubble space telescope to find a TNO to flyby after the dwarf planet. Hubble made short shrift of the work, finding three that the probe could reach. However, the demand for Hubble time does not allow long term searches for TNOs. A survey such as DES will serve to uncover many thousands of more objects in the outer Solar System. As Dr. Michael Brown of Caltech has stated, there is a fair likelihood that a Mars or Earth-sized object will be discovered beyond Neptune in the Oort Cloud.